Cytochrome P450 2D6 (CYP2D6) is involved in the metabolism of approximately 25% of drugs used clinically several of which are commonly used in children of various ages. The CYP2D6 gene has several variant forms that are associated with no activity or reduced activity compared to the normal version of the gene. While genetic testing for CYP2D6 is becoming more accessible, guiding drug therapy with genetic information alone may not be sufficient in children as the genotype-phenotype relationship may be influenced by developmental changes that occur between birth and adolescence. The purpose of this proposal is to investigate the relative roles of ontogeny and genetic variation in the observed variability in CYP2D6 activity in school- aged children and adolescents, and to assess the functional consequences of the observed variability. To achieve these goals, a total of 180 children and adolescents from 6 to 15 years of age, consisting of patients with a primary diagnosis of attention deficit-hyperactivity disorder (ADHD; n=60) and age- and sex-matched controls (n=120), will have their CYP2D6 activity measured every 6 months for 3 years (Aim 1). The activity measurement will utilize the over- the-counter cough suppressant, dextromethorphan or DM, a standard probe for determining CYP2D6 phenotype. The procedure involves administering a 0.5 mg/kg dose of DM and collecting urine over the following four hours. CYP2D6 activity or metabolizer status is assigned based on the relative amounts of DM and the metabolite produced by CYP2D6, dextrorphan (DX), present in the urine sample. Aim 2 will test the hypothesis that CYP2D6 poor metabolizers can be distinguished from extensive metabolizers based on a metabolomic biomarker pattern present in urine samples collected for Aim 1. In the study for Aim 3, the DM to be used will contain a stable isotope of carbon ([13C]) to determine if a rapid, office-based assessment of CYP2D6 is feasible. The principle of this test is that the [13C]-methyl group released by CYP2D6-mediated O-demethylation of [13C]-dextromethorphan appears in expired air as [13C]O2, referred to as a 'breath test'. Finally, the functional consequences of CYP2D6 activity (Aim 4) will be assessed by comparing the systemic exposure to the non-stimulant drug atomoxetine (Strattera(R)) in two groups of ADHD extensive metabolizers drawn from the highest and lowest 20th percentiles of the breath test data distribution. Ultimately, the goal of the research is to personalize the use of medications in children by selecting the appropriate dose of the correct medication for individual patients.